This invention relates to electronic switching apparatus and, more particularly, to electronic switching apparatus wherein a clamping circuit is provided to insure that only a desired input signal is switched to an output terminal as determined by a control signal applied to the apparatus.
Prior art electronic switching circuits are known whereby one of a plurality of input signals is switched to an output terminal in accordance with a particular characteristic of a control signal applied to the circuit. Such switching circuits are advantageous to, for example, control the switching of analog information signals as well as digital or binary signals that might be used therewith. A typical one of such prior art switching circuits which is operated to select one of two input signals to be switched to an output terminal is comprised of a pair of switching transistors having their respective collector-emitter circuits connected in parallel and their respective base electrodes connected to receive the respective input signals supplied to the circuit. These transistors are adapted for emitter-follower operation and the circuit output terminal generally is connected to the common-connected emitters. Control transistors are connected to the respective base electrodes of the switching transistors so as to determine which of the switching transistors is to be conductive, thereby determining which of the supplied input signals is switched to the circuit output terminal.
In the aforedescribed prior art electronic switching circuit one of the control transistors is driven into saturation while the other is rendered non-conductive. The interconnections between the respective control transistors and the base electrodes of the respective switching transistors is such that when a control transistor is driven into saturation, the bias potential supplied thereby to the base electrode of an associated switching transistor is sufficiently lowered to thereby dispose the switching transistor in its nonconductive state. Consequently, an input signal supplied to the non-conducting switching transistor would be inhibited from being coupled therethrough to the output terminal. Now, if the other control transistor is rendered non-conductive, the interconnections between this control transistor and the base electrode of the associated switching transistor is such that the bias potential supplied to that base electrode is of sufficient magnitude to drive that switching transistor to its conducting state. Consequently, an input signal supplied to this conducting switching transistor is coupled thereby to the circuit output terminal. The operation of the respective control transistors is determined by the particular control signal supplied thereto, whereby the particular input signal that is switched to the output terminal by this prior art switching circuit can be reversed merely by suitably changing the characteristics of the control signal.
It is, of course, desirable to produce electronic switching circuits in integrated circuit configuration. However, it has been found that, when the aforedescribed prior art electronic switching circuit is constructed as an integrated circuit, the stray capacitances that are inherent between the respective electrodes of the control transistors as well as the stray capacitances between the transistor electrodes and circuit ground produce a filtering effect such that when the switching circuit is operated, the frequency response characteristic of that circuit is deleteriously affected as a direct function of such a filtering effect.
A further disadvantage in such prior art switching circuits resides in that the switching transistor which is provided with a bias potential of sufficient magnitude to render the switching transistor non-conductive can be driven to its conducting state if the input signal supplied thereto admits of a large amplitude. In that event, an undesired input signal is switched to the circuit output terminal, resulting in erroneous operation of the switching circuit and substantially obfuscating the results obtained by further apparatus.